A hydraulic brake booster provides a power assist to a motor vehicle master cylinder to effect braking. The power assist is provided by a piston which is responsive to increasing pressures in a pressure chamber formed by a hydraulic brake booster housing. A spool valve within the housing communicates pressurized fluid from a power steering pump to the pressure chamber and the piston moves in response thereto to actuate the master cylinder.
Generally, the hydraulic brake booster housing carries a sealing ring in a groove in a housing bore which engages the piston to slidingly seal the piston to the housing. In view of the difficulty in machining the housing groove and keeping it clean before the sealing ring is inserted in the housing groove, it is desirable to provide a groove on the piston, which carries the sealing ring. However, with the housing bore exposed to atmosphere and the piston and sealing ring sliding on the bore surface, it is also desirable to provide a shield which protects or isolates the housing bore from the atmosphere.